1. Field of the Invention
This invention pertains generally to the field of aircraft altimeter systems.
2. Description of the Related Art
A radio altimeter, commonly referred to as a radar altimeter, is a system used for accurately measuring and displaying the height above terrain directly beneath an aircraft. A typical system could be comprised of a receiver-transmitter unit, antenna(s) for transmitting a signal to the terrain and receiving of such signal reflected by the terrain, and an indicator. In some precision approach procedures known to those skilled in the art (for example, Cat II and Cat III precision approach procedures of an instrument landing system (“ILS”)), the use of a radar altimeter system may be required for specifying the exact minimum height above the terrain known as decision height (“DH”). The DH may be considered as a specified altitude in a precision approach procedure, charted as a height above an elevation of a landing threshold point (“LTP”), at which a decision must be made either to continue the approach or to execute a missed approach procedure.
When an aircraft is approaching a runway, the radar altimeter is not able to provide instant height above the LTP (“HATh”) information because the LTP is not beneath the aircraft. Although instant HATh information may be determined by using altitude information provided by a navigation system of the aircraft and elevation information provided by a reference point data source (e.g., a database of a flight management system), errors may exist with the altitude information provided by the navigation system.
For example, altitude information provided by a satellite navigation system (“Sat Nav”) may lack the navigation performance accuracy that is required by “Required Navigation Performance” (“RNP”) standards that have been published to establish navigation performance accuracy necessary for operation within a defined airspace. Although the information provided by the Sat Nav may lack navigation performance accuracy required by RNP standards, the geographic information (i.e., latitude and longitude information) provided by the Sat Nav may nevertheless be useful in determining the instant HATh when an airborne radar system is employed. Radar-based reflection data corresponding to the horizontal distance between the geographic position of the aircraft and the geographic position of the LTP provided by a reference point data source may be used to provide instant HATh information. Although an airborne radar system may provide such instant HATh information, such information is typically limited to a periodic calculation at a relatively slow rate because the antenna of an airborne radar system typically scans a full scene outside of the aircraft continuously and repeatedly and not a singular point. For example, the antenna of a weather radar system may be dedicated to providing weather information by scanning the full scene in front of the aircraft. As such, instant HATh information based upon radar reflection data may be useful for a very short period of time.